The present invention generally relates to trailers which are adapted to be hooked to and pulled by tractors. More particularly, the present invention relates to trailers which are suited for containerized shipments, such as those commonly handled by air cargo and/or sea transporters.
Commercially available trailers have changed only slightly over the last thirty years. These trailers comprise a load bed, a kingpin assembly adapted to be engaged by a tractor, one or more sets of wheels, and various peripheral equipment such as lights, bumpers, etc., particularly as required by the Interstate Commerce Commission. Generally, the load bed of a trailer is designed to facilitate the particular load type to be handled, as for example, a load bed or undercarriage adapted to carry a tank for hauling liquids, a flat bed configuration for hauling items secured thereto, or a van configuration for confining loose loads.
Any commercially acceptable trailer or trailering system must be compatible with standard tractors which have devices known to the industry as "fifth wheels", mounted thereon which are adapted to grip and pivot with respect to the kingpins of the trailers to which they are attached. The coupling operation of a fifth wheel to a kingpin merely involves moving the tractor so that the kingpin is guided through a tapered slot in the fifth wheel to an appropriate position, after which a handle on the side of the fifth wheel is pulled to close jaws around the kingpin to complete the tractor-trailer engagement.
Over the last thirty years, the trend in trailer construction has been to lighten the weight of the trailer while maintaining suitable strength characteristics so that heavier payloads could be accommodated while meeting the various weight requirements imposed by each state, particularly concerning the weight carried by each axle and the bridging distance permitted between axles. Accordingly, most trailers which have been constructed in recent years have been built with abbreviated understructures disposed in the vicinity of the rear axles, thereby substantially reducing the weight of the understructure while providing sufficient support in the axle region to handle most loads. Similarly, the use of "sliders" in connection with these abbreviated understructures has gained wide-spread acceptance in the industry. These sliders generally allow a double axle assembly supporting the rear of the trailer to be moved relative to the underside of the trailer for a distance of approximately nine feet (for a forty foot trailer), so that, depending upon the particular load and state weight requirements, a load may be appropriately balanced between the tractor and trailer. In order to reduce tread wear and increase the efficiency of a given trailer when that trailer is being pulled in an empty or partially loaded condition, the industry has also, when appropriate, utilized axles which may be selectively retracted vertically away from the road surface to reduce the number of tires in contact with that surface when that axle is not needed.
In spite of the approaches discussed above, most trailers presently in use are subject to certain drawbacks which result in less efficient load transportation and higher maintenance and service costs. In particular, virtually all trailers now in use are equipped with landing gears which, during the hauling thereof, are folded under the trailer body. These may be lowered to support the front end of the trailer when that end of the trailer is not supported by a tractor. Since the trailers themselves are quite heavy, landing gears are normally powered by variable speed winches which move the landing gears into position. Although vital when needed, landing gears are basically superfluous to the operation of each trailer during hauling. It has not been economic, therefore, to construct landing gears which are so rugged and durable as to adequately support certain trailers in the fully loaded condition. Further, these landing gears are particularly prone to failure or damage during the hook-up operation when a tractor backing into the trailer to grab the kingpin may exert substantial torques on the landing gear.
Very recently, the above described problems of cargo shipment have been complicated by the increasing use of containerized shipments, and particularly shipments of containers having dimensions approximately ten or twenty feet long and the full width of a trailer body. While these relatively large containers have gained widespread acceptance in the air and sea transport industries, they have created considerable difficulties for truckers who normally deliver the contents of such containers locally, as for example, to standard loading docks where final unloading of the transported materials is to take place. Conventionally, twenty foot containers are provided with a single set of cargo doors disposed at one end thereof. Accordingly, as a standard procedure, many haulers have adapted twenty foot cargo beds to receive such containers, to buckle those containers to those beds and to transport each container individually to its destination.
While it may appear that a conventional flat bed, forty foot long trailer could be adapted to handle two twenty foot trailers, in reality this configuration is not feasible if it is desired that the contents of each of the containers are to be unloaded while the containers are on the bed, as is normally the case. This is because cargo access at the end of at least one of the two containers is restricted by door placement. To attempt to unload a twenty foot container from the side is not economical since almost all loading docks are adapted for end loading trailers.
Very recently, one approach which has been taken by Seaboard World Airlines is the provision of two coupleable chassis, each of which is intended to receive a single twenty foot cargo container. Essentially, each chassis is a twenty foot trailer comprising a load bed, landing gear, and axle assembly which is mounted on a sliding sub-chassis. Each of these trailers may be coupled together to be pulled by a single tractor according to the following procedure a first of the trailers having the container positioned thereon is appropriately located with the landing gears and struts in the down position while the second trailer is coupled to a trailer and the landing gear put in the up position. The wheels on the second trailer are then locked and the cab moved forward so that the sub-chassis and wheels of the second trailer extend backward beyond the end of the second container. The chassis may be operated in this position or alternatively may be backed into the first trailer so that the kingpin of the first trailer will engage with the locking guides in the extended sub-chassis of the second trailer. The landing gear of the first trailer may then be moved to the up position after which the slider on the second trailer is again released and the entire unit moved forward until the wheels of the second trailer have extended even further into a correct position for a one hundred and twenty inch tandem setting, whereupon the slider of the second trailer is again locked and the slider of the first trailer released and brake set. The entire unit may then move backwards relative to the wheels of the first trailer until the two suspensions automatically unite into a load equalizing tandem wherein the axles of the first and second chassis are adjacent to each other.
As seen from the above description, while the coupleable trailer approach will facilitate the transport of two twenty foot containers by a single tractor, this benefit is achieved at the expense of a relatively elaborate coupling procedure which requires the tractor operator to leave the cab five times to couple or un-couple one trailer from the other. Additionally, in order to effect delivery of the contents of the containers, it is necessary to go through the coupling and uncoupling procedure many times in order to facilitate access to each of the containers. Accordingly, although this system has achieved some success, the complexity of this system and its susceptibility to damage, particularly landing gear damage, during the coupling and uncoupling operation, has limited its widespread application in the industry.
When it is desired to haul greater loads than may be easily accommodated on a single trailer, the industry has occasionally resorted to the use of "dogs", which are additional trailers, usually with pivotal front axles, attached in tandem to the preceding trailer, usually through a hook-and-eye or ball hitch arrangement. While this type of transport has achieved some success, its use has been severely limited by safety problems as well as by difficulties in distributing loads between the front and rear trailers and in handling the rear trailer, as for example, during unloading and tight-quarter maneuvering.